Thick film dielectric electroluminescent displays (TDEL) provide a great advance in flat panel display technology. TDEL displays comprise a basic structure of, in sequence, a substrate, an electrically conductive film layer to form the lower electrode, a thick dielectric film layer, a phosphor film deposited on the thick film layer, and an optically transparent but electrically conductive film to form the upper electrode in the structure.
Various aspects of manufacturing TDEL displays are described in Applicant's co-pending U.S. patent application Ser. No. 09/747,315 filed Dec. 22, 2000; Ser. No. 09/761,971 filed Jan. 17, 2001; Ser. No. 09/867,080 filed May 29, 2001; Ser. No. 09/867,806 filed May 30, 2001 and Ser. No. 09/880,410 filed Jun. 13, 2001 as well as in Applicant's U.S. Pat. No. 5,432,015 and International Patent Applications PCT/CA01/01234 and PCT/CA00/00561. The disclosure of these aforementioned applications and issued patent are hereby incorporated by reference in their entirety into the present disclosure.
TDEL displays provide for several advantages over other types of flat panel displays including plasma displays (PDP), liquid crystal displays (LCD), thin film electroluminescent displays (TFEL), field emission displays (FED) and organic electroluminescent devices (OLED). For example, TDEL displays provide greater luminescence and greater resistance to dielectric breakdown as well as reduced operating voltage, as compared to other types of flat panel displays, such as TFEL displays. This is primarily due to the high dielectric constant of the thick film dielectric materials used in TDEL displays which facilitates the use of thick layers while still facilitating an acceptably low display operating voltage. The thick film dielectric structure, when deposited on a ceramic or other heat resistant substrate, may withstand higher processing temperatures than TFEL devices, which are typically fabricated on glass substrates. This increased temperature tolerance facilitates annealing of subsequently deposited phosphor films to improve their luminosity and stability.
Unfortunately, thick film electroluminescent displays have not achieved the phosphor luminescence and colour needed to be fully competitive with cathode ray tube (CRT) displays, particularly with recent trends in CRT specifications to higher luminescence and higher colour. Some improvement has been realized by increasing the operating voltage of the displays, but this increases the power consumption and decreases the reliability of the displays.
Further improvement in the performance of thick dielectric electroluminescent displays was realized through improved phosphor performance. For example, it is disclosed in Applicant's co-pending U.S. patent application Ser. No. 09/798,203 filed Mar. 2, 2001 (the disclosure of which is herein incorporated by reference in its entirety), that an europium activated alkaline earth containing sulfide selected from the group consisting of thioaluminates, thiooxyaluminates, thiogallates, thiooxygallates, thioindates, thiooxyindates and mixtures thereof, can be employed to provide a high luminescent blue phosphor and, thus, improve phosphor performance.
In another example, improved phosphor performance was achieved, in part, by reducing phosphor damage as a result of patterning phosphor films, such as the blue phosphors mentioned above, before they are fully formed and stabilized. For instance, a photolithographic process is described in Applicant's co-pending International Patent Application PCT/CA00/00561, (the disclosure of which is hereby incorporated by reference). A patterned phosphor film is fabricated by first depositing a uniform phosphor layer, using electron beam deposition, then patterning the phosphor film by applying a photoresist. The photoresist is exposed to a selected pattern of light using a suitable exposure mask and then selected portions of the phosphor film are etched away using a suitable etchant in order to form the required pattern. The patterning is done prior to heat treatment, when the phosphor is in a more reactive and, hence, etchable state. The problem, however, with using such a method is that the deposited patterned phosphor film is susceptable to reacting with the etchant and other chemicals used in the photolithographic process, which causes a reduction in the fully processed phosphor performance and stability. To eliminate this type of degradation, another process was used whereby the phosphor film is deposited using a shadow mask such that patterning is achieved during deposition of the phosphor film rather than after the phosphor film is deposited. This method, however, is generally not applicable to high-resolution displays whereby the required spatial definition of the patterned film exceeds that which can be achieved using shadow masks.
Other techniques to patterning films have been described in U.S. Pat. Nos. 4,970,366 and 4,970,369. These patents are directed to laser patterning of transparent electrically conductive layers in displays such as liquid crystal displays.
It is therefore desirable to improve the performance of thick dielectric electroluminescent displays, particularly the luminescence and energy efficiency of these displays, by offering a process for patterning thin films in thick dielectric electroluminescent displays which overcome the limitations of the prior art.